The present invention is generally directed to a calibration method, and more particularly to a method for calibrating a system adapted for spectrographic analysis of gaseous substances.
Instrumentation that analyzes gaseous substances is required in a variety of important applications. For example, during a surgical operation, patients are anesthetized through the careful administration of gases such as nitrous oxide. The supply of these anesthetics must be regulated with great precision. In addition, the gases expelled in the patient's breath need to be monitored continuously to determine the condition of the patient. Instrumentation that analyzes gasses exhaled by patients provide vital information to surgical personnel. Field calibration of such instrumentation maintains its accuracy while maximizing its availability for operating room procedures.
A particular class of instrumentation employs Raman spectroscopy to detect the presence and concentration of gaseous substances. Scattering of light by the Raman effect has received much attention from scientists since its original exposition by C. V. Raman in 1928. Instrumentation that employs the Raman effect provides a light, such as a laser beam, which illuminates molecules of a gas disposed within a sampling cell. Molecular vibrations of the gas cause the light to scatter off the illuminated gas molecules to produce scattered light in a process which shifts the frequency of the scattered light by exactly the vibrational frequency of the molecule. More generally, the scattered light comprises a spectral signal generated by stimulating the gas with the light. The frequency shift of the spectral signal is characteristic of the gas being analyzed and is independent of the frequency of the illuminating light. Thus, measurement of the spectral signal can be used to infer properties of the gas being analyzed, such as chemical composition and concentration. The spectral signal is collected from the gas disposed within the sampling cell and the constituent frequency components of the spectral signal are analyzed. The analyzed frequency components are used to produce a spectrogram, which can be displayed on a display device. By interpreting the spectrogram, the presence and concentration of different types of constituent gas molecules in the gas can be deduced.
Instrumentation that employs the Raman effect is described in U.S. Pat. No. 4,648,714 entitled "Molecular Gas Analysis By Raman Scattering in Intracavity Laser Configuration" by Benner et al. issued Mar. 10, 1987, and in U.S. Pat. No. 4,784,486 entitled "Multichannel Molecular Gas Analysis By Laser-Activated Raman Light Scattering" by Van Wagenen et al. Because each of these patents provide helpful background information, they are incorporated herein by reference. The instrumentation described in each of these two patents provides a laser beam that illuminates molecules of a gas disposed within a sampling cell. Raman scattered light generated by the Raman-effect is collected from the gas disposed within the sampling cell. Each patent focusses on a respective means for detecting constituent frequency components of the Raman scattered light. As disclosed in each patent, a microprocessor coupled to the detection means receives electrical signals representative of the constituent frequency components of the Raman scattered light. Results are displayed on a display controlled by the microprocessor.
It should be briefly noted that although it is possible to display a given spectrogram in any one of a variety of different formats, the substance of fundamental calibration requirements discussed herein remains, regardless of display format. The fundamental calibration requirements discussed herein can be suitably translated for each different format. For the sake of clarity, a common spectrogram format is discussed herein. For each type of constituent gas molecules, the spectrogram displays a single vertical spectral peak or several vertical spectral peaks located along a horizontal axis of the spectrogram. Location of spectral peaks along the horizontal axis of the spectrogram is determined by the amount that the frequency of the spectral signal is shifted from the frequency of the illuminating light. The amount of frequency shift is scaled by the speed of light and displayed along the horizontal axis of the spectrogram as a wave number offset. Location or "height" of spectral peaks along the vertical axis of the spectrogram corresponds to relative intensity of the spectral peaks. Relative intensity of the spectral peaks is determined by the respective relative concentration of each different type of constituent gas molecule.
As discussed herein, there are fundamental requirements for field calibration of instrumentation for Raman spectroscopy. A first field calibration requirement is to correctly scale spectrograms displayed by the instrumentation. This first calibration requirement includes displaying spectral peaks of any given spectrogram at correct relative intensity locations along a vertical axis of the given spectrogram. This first calibration requirement further includes displaying spectral peaks of the given spectrogram at correct frequency locations along a horizontal axis of the given spectrogram. For example, it is expected that a spectrogram of nitrous oxide, N.sub.2 O, produced by properly calibrated instrumentation should display spectral peaks located along the horizontal axis of the spectrogram at wave number offsets of 1285 and 2224 cm.sup.-1. To be effective, a calibration method should operate over a broad range of frequencies.
A bottle of a pre-measured calibration gas mixture containing a wide range of constituent gasses at premeasured concentrations is useful in satisfying this first requirement. During field calibration, the bottle is opened and the gas mixture is released into the sampling cell for analysis. The instrumentation produces a spectrogram having a wide range of spectral peaks due to the wide range of constituent gasses. Since the identity of the constituent gasses disposed within in the bottle is already determined, correct frequency locations of spectral peaks in the spectrogram of such gasses is already determined. The correct frequency locations are used to align the frequency locations of measured spectral peaks along the horizontal axis of the spectrogram. Since concentrations of constituent gasses disposed in the bottle are already determined, correct relative intensity levels for spectral peaks in the spectrogram of such gasses are already determined. The correct relative intensity levels are used to adjust intensity gain of the instrumentation with respect to the measured relative intensity levels of the spectral peaks.
A second field calibration requirement is to reduce negative effects of collateral scattering on the spectrograms displayed by the instrumentation. Though the phenomenon of collateral scattering is not fully understood, it is theorized that collateral scattering takes place when the light source which illuminates molecules of the gas being analyzed also illuminates other materials adjacent to the gas, thereby causing the adjacent materials to scatter the light or to fluoresce. Such light from the adjacent materials is undesirable and is referred to herein as collateral scattered light. It is theorized that the spectral signal, comprising desired fight scattered from the gas being analyzed, becomes intermixed with the collateral scattered light. This intermixing degrades the spectral signal collected by the instrumentation, which in turn obscures the spectrograms displayed by the instrumentation.
A bottle of a purified noble gas, such as purified Argon, is useful in satisfying this second field calibration requirement. During field calibration, the bottle of noble gas is opened and the noble gas is released into the sampling cell for analysis. It is theorized that only light from the adjacent materials is collected because noble gasses do not exhibit Raman scattering. Accordingly, the instrumentation generates a background signal spectrogram that is representative of the collateral scattered light. The background signal spectrogram is then used to reduce the negative effects of collateral scattering on the spectrograms displayed by the instrumentation.
As discussed, specialized calibration gasses such as the purified noble gasses and the pre-measured calibration gas mixture are useful in calibration. However such specialized calibration gasses also have disadvantages. For example, completing calibration using these gasses naturally requires availability of both the bottle of noble gas and the bottle of pre-measured gas mixture. Since analysis of the noble gas does not produce a spectrogram that is useful in frequency location alignment or gain calibration, the pre-measured gas mixture would be needed to complete calibration. Similarly, since analysis of the pre-measured calibration gas mixture produces a spectrogram having numerous spectral peaks that obscure the background signal spectrogram, the noble gas would be needed to complete calibration. By this reasoning, a hospital organization that performs field calibration would need to maintain a supply of bottles of pre-measured calibration gas mixture as well as a supply of bottles of purified noble gas.
Every item stored and dispensed by a hospital for a patient's surgery directly or indirectly adds to the cost of the surgical procedure. Skyrocketing health care costs require simplification and streamlining of medical procedures, while maintaining high quality care. While field calibration is needed to maintain the integrity of spectrographic measurements, the use of specialized calibration gasses adds to cost and complexity of field calibration procedures. A new field calibration method is needed that satisfies the field calibration requirements while eliminating the expense and inconvenience of specialized calibration gasses.